1. Field of the Invention
This invention relates to offshore platforms and vessels. More particularly, it relates to floating structures which employ porous, added-mass stabilizer plates for motion suppression.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98.
U.S. Pat. No. 3,986,471 describes an apparatus for damping vertical movement of a semi-submersible vessel having submerged pontoons and a small waterplane area which comprises a submerged damper plate equipped with valves for providing substantially greater resistance to upward movement of the plate than downward movement. The damper plate is supported deep beneath the semi-submersible vessel by flexible, tensioned supports such as chains or cables, at a depth beneath the water surface in the semi-submerged condition of the vessel where the amplitude of subsurface wave motion is less than the maximum heave amplitude which would be experienced by the semi-submersible vessel alone under identical sea conditions. The area of the damper plate is several times larger than the waterplane area of the vessel. An upward only-damping action is achieved due to the entrainment of large apparent masses of relatively still water by the damper plate.
U.S. Pat. No. 5,038,702 describes a semi-submersible platform supported on columns with pontoons extending between and outboard of the columns. Damper plates are provided by flat surfaces either on top of the outboard section of the pontoons or by plates positioned on the columns above the pontoons to provide heave and pitch stabilization and motion phase control in relation to the wave action such that when the platform is in the drilling mode, the heave phase of the platform is approximately 180° out of phase with wave action, and in the survival mode, heave action of the platform is substantially in phase with wave action.
U.S. Pat. No. 6,652,192 describes a heave-suppressed, floating offshore drilling and production platform that comprises vertical columns, lateral trusses connecting adjacent columns, a deep-submerged horizontal plate supported from the bottom of the columns by vertical truss legs, and a topside deck supported by the columns. The lateral trusses connect adjacent columns near their lower end to enhance the structural integrity of the platform. During the launch of the platform and towing in relatively shallow water, the truss legs are stowed in shafts within each column, and the plate is carried just below the lower ends of the columns. After the platform has been floated to the deep water drilling and production site, the truss legs are lowered from the column shafts to lower the plate to a deep draft for reducing the effect of wave forces and to provide heave and vertical motion resistance to the platform. Water in the column shafts is then removed for buoyantly lifting the platform so that the deck is at the desired elevation above the water surface.
U.S. Patent Publication No. 2002/0139286 describes a heave-damped floating structure that includes an elongate caisson hull and a plate set coupled to the hull. The plate set includes multiple heave plates located about an outer edge of the hull so as to form a discontinuous pattern generally symmetric about a vertical axis of the hull.